CT,VT,CVT

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CT,VT,CVT

• Connections of a CT and a PT to supply, load and
  relay.

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CT, VT, CVT

• The voltage transformers and current transformers
  continuously measure the voltage and current of
  an electrical system and are responsible to give
  feedback signals to the relays to enable them to
  detect abnormal conditions
• CTs and VTs are the eyes and ears of the
  protective system.
• They extract information from the power system
  and form an important link between the
  high-voltage high-current power system and the
  low-voltage low-current protective system. CTs
  and VTs have two important roles to play, namely
• 1 Electrically isolate the protective relays from
  the high-voltage power system for, the purpose of
  safety of the operating personnel.
• 2 Step down the current and voltage to standard
  values of 1 A, 5 A, 110 V so that the design of
  relays can be standardized irrespective of the
  actual primary voltage, and current.

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Current Tranformer

• There are two types of current transformers
• 1. Wound primary type
• 2. Bar primary type.
• Bar primary
  
  Wound
  primary
• The wound primary is used for the smaller
  currents, but it can only be applied on low fault
  level installations due to thermal limitations as
  well as structural requirements due to high
  magnetic forces.
• For currents greater than 100 A, the bar primary
  type is used
• Protection CTs are most frequently of the bar
  primary, toroidal core with evenly distributed
  secondary winding type construction. In such CTs,
  the secondary winding can be conveniently put
  inside the high voltage bushings.

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Current transformer

• The basis of all transformers is that
• Amp-turns on the Primary Amp-turns on the
  secondary
• e.g. 100 A 1 turn 1 A 100 turns
• The primary current contains two components
• An exciting current, which magnetizes the core
  and supplies the eddy current and hysteresis
  losses, etc.
• A remaining primary current component, which is
  available for transformation to secondary current
  in the inverse ratio of turns.

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• The exciting current is not being transformed and
  is therefore the cause of transformer errors.
• The amount of exciting current drawn by a CT
  depends upon the core material and the amount of
  flux that must be developed in the core to
  satisfy the output requirements of the CT. that
  is, to develop sufficient driving voltage
  required, pushing the secondary current through
  its connected load or burden. This can be
  explained vectorally as shown in figure

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Vector diagram for a current transformer
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• The transition from the unsaturated to the
  saturated region of the open-circuit excitation
  characteristic makes a CT not to produce
  equivalent primary current beyond certain point.
  This transition is defined by knee-point
  voltage in a CT, which decides its accurate
  working range.

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Equivalent circuit of CT
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Equivalent circuit of CT
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Phasor diagram of CT
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CT errors
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Phase angle error
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• The conventional CT core saturates during
  transient causing delay in high speed relay
  operation
• Secondary resistance
• The secondary resistance of a CT is an important
  factor, as the CT has to develop enough voltage
  to push the secondary current through its own
  internal resistance as well as the connected
  external burden. This should always be kept as
  low as possible.
• Burden circuit connected to secondary winding.
  Expressed in volt-ampere at rated secondary
  current at rated power factor.
• CT specification
• A current transformer is normally specified in
  terms of
• A rated burden at rated current
• An accuracy class
• An upper limit beyond which accuracy is not
  guaranteed (known as the accuracy limit factor,
  ALF), which is more vital in case of protection
  CTs.

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• In terms of the specification a current
  transformer would, for example, be briefly
  referred to as 15 VA 5P20 if it were a protection
  CT or 15 VA Class 0.5 if it is a metering CT. The
  meanings of these figures are as below

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Open circuits of CTs

• Current transformers generally work at a low flux
  density. Core is then made of very good metal to
  give small magnetizing current.
• On open-circuit, secondary impedance now
  becomes infinite and the core saturates. This
  induces a very high voltage in the primary up to
  approximately system volts and the corresponding
  volts in the secondary will depend on the number
  of turns , multiplying up by the ratio (i.e.
  volts/turn no. of turns).
• Since CT normally has much more turns in
  secondary compared to the primary, the voltage
  generated on the open-circuited CT will be much
  more than the system volts,leading to flashovers.
• HENCE AS A SAFETY PRECAUTION, NEVER OPEN-CIRCUIT
  A CURRENT TRANSFORMER ON LOAD!!!

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Voltage (potential)Transformer VT or PT

• Voltage transformers are much like power
  transformers operating on very light load.
• There are basically, two types of voltage
  transformers used for protection equipment.
• 1. Conventional two-winding, electromagnetic type
  (commonly referred to as a VT)
• 2 Capacitive voltage divider type(referred to as
  a CVT).
• The electromagnetic type is a step down
  transformer whose primary (HV) and secondary (LV)
  windings are connected as below

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• The number of turns in a winding is directly
  proportional to the open-circuit voltage being
  measured or produced across it. The above diagram
  is a single-phase VT.
• In the three-phase system it is necessary to use
  three VTs at one per phase and they being
  connected in star or delta depending on the
  method of connection of the main power
• source being monitored.
• This type of electromagnetic transformers are
  used in voltage circuits upto 110/132 kV.

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Capacitive voltage transformer (CVT)
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CT Testing